LTE (Long Term Evolution) of which the specifications are designed in 3GPP (3rd Generation Partnership Project), which is a project aiming to standardize a cellular communication technology, supports a cellular/WLAN radio interworking technology in Release 12 and later (see Non Patent Document 1 and 2). With such a technology, a user terminal in an RRC connected state or an RRC idle state performs a bidirectional traffic switching (network selection and traffic steering) between an E-UTRAN and a WLAN.